US1533311A - Secret signaling - Google Patents

Description

April 14, 1925. 1,533,311

` H. FLETCHER -SECRET SIGNALING Filed Deo. 1s; 1923 OTO / (5f. /6/ g? D 8550;" my;

@y Mfr/Wy Patented 'Api'. 1 4, 1925.

HARVEY FLETCHER, or NEW YoRx, N. Y., AssIGNo-R To wEs'rERNnLEcrRm com:F

PANY, INcoRroRATED, or NEW YoRx, N. Y., A coRPoRA'rIo'N QFNEW YORK.

sEoRnr s Application led VlDecemyher To all who/m, it my concern."

Be it known that I, HARVEY FLETCHER, a citizen of the United States of America, re-

siding at New AYork city, inl the county of- 5 New York, State of New York, have invented certain new and useful Improvements in Secret Signaling, of which the follow- -ing is a full,I clear, concise, and exact descri tion.

lo T e present invention relates to wave Y transmission with secrecy. f

The invention is applicable to the art 0f signaling and to the transmission of electrical waves for any purpose.

15 An object of the invention is to secure a high degree of secrecy in a signaling o r other` wave transmission s stem while at the same time maintaining til the system and the qualityf the reproduced waves.

It has been proposed to divide the waves that are to be transmitted into subbands of frequency components and to render the waves diiicult of recognition by transposing certain of the sub-bands of components prior 'to the transmission of the wave.

' Such transposition may comprise interchanging two or nore of the sub-bands as to the frequency positions normally occupied by the-respective sub-bands, or it may comprise inversion of the frequency order-0f the components ofone or more of the sub-bands either with or without an accompanying interchange being made between the frequency positions of the'sub-bands. Still other transformations for the waves have been pro- From the standpoint of--f'adaptability of a secrecy system tostandard transmission cir- 40 cuits, it is desirable that the altered waves -whichlare actually transmitted occupy the same absolute frequency range as -the waves would occup if' left in .their normal or unalteredcondltion From the standpoint of secrecy, it is desirablethat transformations made in the waves. be such as will produce ymaximum confusion when -an attempt is made'to receive them on any ordinar and unauthorized receiver. From the stan point of economy in transmitted` power it is desirable that no noise components be added and that no' components other than those derived from the waves themselves beV sent.

e general '-eiiiciency of IGNALING.

1s, 192s. Serial'No. 681,346.

The invention comprises, among its' features, provisions for securing each /of the above-mentioned desirable results.

vThe various objects' and features of the invention will more fully appear from the description now to be given of the embodinient illustrated in the accompanying drawmff. 4 ln this drawing, Fig. 1 shows in symbolic form the manner of (lvision of the normal speech into sub-bands a, b and c and the mam nerof tansposing the sub-bands prior to transmission according lto the invention.-

Figs. 2 and 3 show, in schematic orm,la

transmitting system and a-receiving system,

respectively, for carrying 'onsecret signaling in accordance with the invention..y v

It has been found that the intelligibility of a sub-band of frequencies divided out of y the speech band depends not only on the breadth of the sub-band but also upon the region of the speechl band from which the sub-band is taken.

It is also found 'that the energy sented in any sub-band of speech of given frequency width diers markedly depending upon the region of the speech bandfrom -which it is taken. The degree of intelligibility of a sub-band does not as arule correspond at all closely with the energy 4which itl possesses, so that it is possible yteg-divide 1- fnormal speech into sub-bands of the same frequency width and same energy content but of widelydifferent degrees of-intelligi bility, and, further, sub-bands of different frequency width and the same intelligibility may'diifer widely in energy content.

sii,

As "might be expected from what has justv been said ,relative tothe effects of venergy content andfrequency position of sub-ban it has been found that the' amount of cony fusion that may be produced by'asub-band of noisecurrents of givenfre' and energy content, differs. wide y..accord1ng as the noise-components are introduced into one region or anotherof the normal speech range. Smce. any selected` sub-band of speech can be converted into nolse'or cono5 uency with v fusion components bymaking some trans'-- position of the components, either by inverting theirorder within the sub-band or -by shifting. `the'- mfrequency position of the los` sub-band', 'the 'total @olif-1181.011"pro'ducedlfroml j so operating on a sub-band depends markedly upon the width of sub-band that is selected,'the re ion of the normal speech band from whic it is taken and the nature of the transposition made of it.

For a fuller' discussion of the relations between 'band width, ener content and intelligibility of portions o the speech band, reference may be had to applicants paper on The Nature of Speech and its Interpretation, published in Journal of the Frank"y y llin Institute for dune 1922, Vol. 193, No. 6.

- In dividing the speech band into subbands for secret transmission in accordance with the invention, advantage is taken of the dependence of the degree of intelligibility upon the frequency width, the fre uency p0- sition, and the energy content of t e various sub-bands that may be derived from normal speech. Also, in making the required transformations in the transmitted waves in order to produce confusion, 'the invention band c in spiteof its 'greater fleqluencysub-band' Zrmay extend from 800 to1'600 cycles and the sub-band c from 1600 to 3000 cycles. On this basis, the a sub-band has anintellligibility of about 16%, 'the b sublband `27% and the c sub-band 25% while the voice energy represented by the asubband is aJbofut72%, that of the b sub-band is 18% and that of the c sub-band is 6%. Each sub-band by itself is seen to be highly unintelligible since it enables only one sound in four, or one sound in six in the case of the a sub-band,to be understood. With this scheme of sub-division, it will be seen that the same sub-band width has greateriintelligibility-in the b position than in the'a position and that the intelligibility in the sub'- width is lonly of the .same order as at of the sub-band` Ib. It will also be seen that the su -band al of the least intelli ibility has the eatest energy content an that the smal .energy handle although this sub-'band possesses the greatest frequency width.

Sub-bands a and b being of .equal freuency width readily permit of being interc anged in their; ,frequency position. The three possi-ble .transformations on this basis of sub-divisionfare shown oppositethe rev- 'spective number slhxin Fig'. f1. The 'com-, V blnation'4 changing the the mod ulatin will be not Von y to reproduce the sub-band content is that of the subfrequency position of the sub-bands a and b. The second combination is obtained by shifting the sub-band b to the a position and by shifting the sub-band a to the b position and inverting its frequency order as is shown by the reversal of the arrow. The third combination is obtained by shifting the subband a to,the'b position and shifting the sub-band b to the a., position and inverting its frequency order.

If an attempt is made to receive any of' the combinations 1, 2 or- 3 in an ordinary telephone receiver the intelli'bility will be` practically nil. lt is-foun that merely 'shifting a sub-band of the speech frequencies to a higher or lower frequency position without disturbing the frequency order within the sub-'band renders the sub-band highly unintelligible. In each of the three combinations therefore, about 90% of the received sound energy comprises confusion or noise components so that the small intelligibility of the sub-band a is completely overwhelmed by these noise components.

If an attempt is made to break the secrecy of these combinations by using a l heterodyne oscillator, thefreuency of which may be varied throughout t e range of the received frequencies the best than can be accomplished will Ibe the restoration of one of the sub-bands to its normal position in the s h range butthis restored sub-band will iieec ber ofy confusion or noise components.A For example, let it be su posed that a 'heterodyne oscillator' is a )usted to receive the sub-'band a of the first combination by havin itsirequency set at 800 cycles. If the su -bands of combination 1 are used to modulate a' carrier -wave the heterodyne oscillator'would, of course, vhave-its frequency ave superposed upon it a large numl made equal to the corresponding component -in the side band, that isf-thecomponent produced from the 800-cycle,l component in wave. The effect of this a in its normal position 'in the lspeech range but also to superpose-thersubfband b upo the sub-band a and to displace the'sub-band.

c downward from its normal vposition by 800 cycles. Both t e sub-band b and the sub-band etherefo a pear as noise currents. Further, the su band b cannot -be separated from the sub-band a by `filteringy since thev occupy the same frequenc ran If-theyheterodyne oscillator be seyt to ceive the sub-band b of combination 1 prop# erly, both the sub-bands a and c are displaced upward in the frequency scale 800 cycles from their combination position and will, therefore, appear as noise currents. On account of the greater energy content of v-the sub-band a the noisel produced b itwill be much greater than the volume'o the restored sub-band b.

the restored sub-hand..

It has been assumed above that the scheme of transposition Vof thetransmitted wave is In practice, how-ever, these conditions would-V rarely, lf'ever., 'exist and the chances of obwhich are niet i bilitylaccurate tuningwith an intelligit blhtg so low is Very small.

:und 3 which illustrate one form of carryin rent derived from 3'O phone 10 are transmitted throu h the reknown to the unauthorized-.observer and that he is able to locate the frequency` of the heterodyn'e oscillator exactly at the desired point in the various frequency combinations.

taining even a`s good intelligbilit as is assumed above; would be very smal, first, because there will alwaysvbe other distortions l considered in the above dis cussion, 'and secondly, because the proba- Reference will 'now be made to Figs. 2 g out t invention. In Fig-2 the speech curl the circuit of the micropeating coil 11 to the three filters 1, F2 and F3 which serve to divide the speech currents into three sub-bands a,b and c as indicated of 800 to 1600 cycles and 40 in Fig. 1. The filter Fris designedl to transmit selectively the compoxents 0 to- .800\ cycles" and to ei'ectiyely suppress components of lall other frequencies. 'Similarly,{ the lilterl2 transmits `only the components the filter F3 transmitsonly components of 1600 to3000, cycles as well asthe other filters employed inthe system.. Each of the@ filters may be of the type disclosed'in U." S.' patent to Campbell N0. 1,227,113, May 22, 1917. .i

The output Vof. the filter F,l is passed through a repeating coil 12 and is applied differentially to the grids of the two tubes of the 'balanced modulator Mx. A' source of waves ,13 is applied from thegrids of these tubes in parallel through the 'transformer 14. The resulting modulation components are `transmitted vthrou h the output transformer 15 tothe outgomglter F4. The fil.` ter F4 is designed'to .have a transmission range of 800 to 1600 cycles and to suppress the transmission through it of components of all otherfrequencies. The frequency'of ythe sourse 13 depends-upon the lparticular frequency transformation that is to be pro duced inthe sub-band a. If this sub-band is to be shifted as in .combination l 1 of. Fig. lso that it occupi/es the subfba-nd' b posltlon, source 13 willl have a frequehcyofSOO cycles.

-The modulation com'ponentswill comprise a'n upper and a lower side` band,.the upper side band extending from 800 cycles to 1600 cycles. This side band will beilselectively transmitted by the filter F4 while the other side band will be completely ysu pressed." l

Since the transmission ranges of t e filters F1 and F4 do not overlap there will be no direct transmission through the 'modulating system of any i unchanged. A The modulator circuit M1 is eral type disclosed in'U. S. l.patent to Carson No; 1,343,306, June 15,1920. i

The output of the filter F2 isfimpressed..

f thc genl l.

of the a sub-band frequencies on a second balanced modulator'M., which,

may be of the same type as themo'dulatory M1 but which, for simplicity, in the drawing, is merely indicated by a rectangle. This modulator is supplied from a 'source 16, with a wave, the frequency of lwhich depends upon the 'particular transformation that is to be made inA the outgoing wave'.Y If the b sub-band is tobe shifted to the wsub-'band position as in combination 1 ofFig. '1, the

source 16 will have a frequency off800 cycles.

As a result of the action ofthe"modulator circuit M2., side bands will be produced,I one frequency between the applied b sub-band and the frequency displaced downward in frequency by 800 400 of which is' represented by the 'difference in of the source'16, that is',-

-each component of the b ysub-band will bev cycles so that the resulting side b'andwill l occupy the frequency range, 0 to S00 cycles. This side band will' be selectively trans-V mitted through the filter F5 to the exclusion of components -of all otherifrequencies.l Since no shift is to be made in the frequencies of the sub-band c, no modulator 1s required, but the filter F,l has its' output connected in common with the outputs of filters F4 and F6 to the common repeatin coil 17.

If desired thesub-bands from` t 1e. filters F-3,F4 andFs'could be transmitteddirectly over a telephone line or other circuit to the distant 'receiving station. These sub-bands would in 'such case appear merely asunin-i telligihle noise4 -in any ordinary 'receiver 'which might be associated with the hne slnce4 they .representa normal speech wave trans- A formed in accordance withcoinbination-l of Fig.'- 1, the effect of: which has been 'discussed above. i

The particular type of transmission cn',- cuit that has been chosen for illustration, however, isa .radio transmission system, so

'that the transformed speech wave is used in the system illustrated to modulate aradio f requencywave which ma;` belderived from' I anysuitable'oscillator 1 8. The'r'atlio waves are a plied in parallel to the` grids of the modu ator M and the secret speechwaves are vapplied dili'erentially to the modulatorgrids. As a result, the unmodulated carrier I component from source 18 is suppressed and only the two side bends of the secret s eech based on the radio wave are pro uced.

`aide bands are impressed on the transmittingantenna TA by means of the trans- 1- former19. The antenna TA may be tuned to i transmitselectively one 'of the two side fbandsito -the exclusion of the other. Or, in

:case the radio; frequency is t'oo high to perl mit of -discriminatien1between the side bands.

extending from 800 to 1600 cycles. -Due to `the, .characteristics of thel-iiltcr F'4 only the 2,0-lowe'r side band is transmitted. Since this side. band represents: the ditlerence'- in fre- .quency'rhetweenthe appliedfl-'wavesthe fre- JAquen'cy order within'this'side ban'd'isl the reverse of that ofthe applied a sub-band.

,To'obtain the necessary shiftofthe subband b for vcombination 2, the source 16 is made have a frequencyof 800 cycles as '1n 'thecase of combination' 1.".

e source 16 is. made to have. a frequency l yof 160Q cycles. `-Inthis 'case an upper side band, extending' from 2400't'o 3400 cycles is i. obtained and a'lower side band extending 4latter of vthese two. side lbands* is' transmitted through the filter. F:s to thefoutgoing cwcuiux The Wave trzinsmitted from the station. of

IRv which betunedto the frequency of 'the wavestolbe received. .The waves picked 'uP by4 thlantenna RA are transmitted-into y limpressed on. the detectopD lthe.-inpu{; 'of

'the tuned circuitQO 'from which they .are

. which is also supplied with a wave of the radio orcarrienfrequency from source 21.

i f As a-'rcsult off-.the interactionin the detest'- o ing circuit Aof the received-Smetana ,with the locally, supplied carrier' wave, a 'band through the filter F3', u

"so impressed on the receiver represent the .To obtain ,combination of' Fig. l, the 39 sourcel is made to have a frequency of 800 `goles as.1n"the. case of com ination 1 and'.

therefore, the speech which was to be transv Fi 2 is received on the receiving antennav What is claimed is: lof frequency. components is produced ofthe same character, as those used to modulate' the carrierr at. -th1 a` transmitting. station.y components are ,divided into sub-- bandsjby-the filters Fl, Fz-nnd-GF," iwhic'li-4 may'duplicates respectively of the filters sub-band l1selectivelyz -p''assed by-l-thef .representsthe transposed sub-.band thejqspeechfwaves, and transmitting said 17e-125 lmlpessedfon/theinpt offthe des f' `as the source 16 ofF-ig. 2. As a result of the action of the demodulator circuit DMl, two side bands are produced, the upper ofv which extends from 800to 1600 cycles and represents. the restored sub-.band b. This, sub-band' is selectivelypassed bythe filter FJ to' the exclusion of'componentS, ofl all other frequencies. Thel sub-band from'the 'filterFz represents the transposed sub-band "a and is impressed on fthe` demodulator 4DMz which may be ofthe same type as" the demodulator DM1 and which is supplied with a wavel from source 13 of the same frequency-as source 13 of Fig, 2; The relsultinglower side bamd'occupies the fre-v quency range. 0 vto 800 cycles and 1s selec,-l tively passed by the filter` F4 to the exclusion of components ofjall other frequencles. j The'remalning sub-band representlng the unchanged sub-band c is transmitted direct- "which may be a plicate'of the filter F3, ancl is impressed in commonwith the waves from; the filters F4 and F5 vupon the receiver R.' 'llhe waves 910 restored sub-bands originally derived` from the speech currents transmitted from".the repeating coil 11 of Fig. 2 and represenL'S mitted,

While it is preferable, for the sake of economy in transmitting energy and 1n the frequency range employed where transmissiontakes place yby modulating a ,carrier or 'radio wave to suppress the c rrier and to transmit'only one side'band tiis is not essential' to the invention-L It will be understood from what .is stated above that any' type of transmission, either carrier or radio transmission or direct transmission over a line at 'speech `frequencies ma be' employed. i

ile it hasl been assumed above that speech issub-divided into -only three subbands, two of'which are of the' same fre- -quency width, vit is within the invention to employ more than two sub-bands of the same frequency and to interchange them prior to transmission together with one or more sub-,bands which are unchanged.l 115 1. The method of secretly 'transmitting speech waves comprising dividing the waves -into sub-bands of frequencies, the sub-bands differing among themselves asI to the breadth 4120 of included fre uencies, interchanging cersul'tant waves to a'distance.-l

thod of secret 'transmission' of 2, The

l"speech comprising dividing the normal speech' components v, into' sub-bands including a plurality of 'culi-bands 'of the same fre-1,311'

- quency width `and a sub-band comprising .the higher frequency region of normal having smaller voice energy, interchanging the frequency position Aof certain of said sub-bands with -others while keeping the resultant n'aves Within 'practically the same speech waves, and transmitting the resultant Waves.

4. 'lhe method of secret transmission of speech which comprises dividing normal speech waves into a sub-band of frequency components possessingsmall voice energy f and low intelligibility, and into other subbands possessing low intelligibility but greater voice energy than the first-mentioned sub-band, transposing only the subbands of larger energy contentamong themselves to reduce the intellgibility of the resultant waves below that of any sub-band by Jitselfl, and transmitting the Wave so transpose n Y 5. The method of secret transmission of speech comprising transmitting certain of the frequency components of normal speech in their normal frequency relations, selecting sub-bands of other frequency compon- `cnts from normal speech and interchanging the frequency positions of the sub-bands to produce unintelligible confusion components occupying as a whole the same' fre; quency positions as before their transposition, and transmitting the interchanged components along with the unchanged components. Y f n:

6. The vmethod of .secret telephony coml prising dividinv normal speech waves into fi uency components, eachsub-bands of sub-band by itse ossessing small intelligibility, said subands differing among themselves in energy contentv and in frequency` width, producing noise currents rom certain of the sub-bands of compon len ts'of the? larger energy content'f'ndtransnnttmg the resulting noise 'currents'along `with the unchanged' components -whereby the intellgibility of the wave components as a whole is practically nil, the" transmitted divide normal speec extreme frequency lnnits as the original,

Wave occupying substantially the same frequency limits as the original speech.

Y 7 In a telephone system an incoming circuit, an outgoing circuit, a source of speech Waves in saidincoming circuit, means including filters'for dividing the speech waves into sub-bands of frequency components,

means between .said circuits foitransmitting certain frequency components from said source directly to the outgoing circuit substantially unchanged 4in frequency, means for interchangng the frequency positions of certain sub-bands of components from said source while keeping them as a whole content andy each by itself'possessing small intelligibility, means to transmit substantially unchanged a sub-bandxof relativelyl small energy content and means to interchange the frequency, positions of sub-bands of relatively large energy content and to transmit .the interchangedv sub-bands along wvith the unchanged sub-band.

9. In a secret telephone system, means to modulate a high frequency carrier wave in accordance with the unchangedand interchanged sub-bands produced by the..

means defined in claim 8, means for sup- .pressing one of the side-bands of the resulting modulated vvavev and means for transmitting the remaining'sidefban'd.

10. The method of receiving and rendering intelligible the waves transmitted by the method defined in claim 1 comprlslng dlviding the received waves into res ective subbands composing the wave'that 1s transmitted, and inter'changing in reverse manner the sub-bands that result from interchanging sub-bands of normal speech, to restore the speech. 11. A receivingsystem forreceiving and rendering intelligible'the waves transmitted by a .system as vclail'ned incla-im 7,compr1s 4ing filters for dividing the received waves into sub-bands ofvfrequency components, a 'receiver, means to transmit the components fromone of said lilters directly to said receiver, and means for restoring to normal frequenc position the sub-bands from others o `said filters and for impressing the restored sub-bands on said receiver.

In witness whereof, I hereunto subscribe my name this 12 day of December A. D.,

A.1923. HARVEY FLE'rCHIizR.

Images